‘The Front Runner” traces a turning point in American political journalism back to the 1988 presidential campaign, when the auspicious candidacy of Gary Hart, the Democratic front runner, was destroyed by an unprecedented feeding frenzy over rumors of sexual impropriety. The film, based on a widely admired 2014 book by Matt Bai, is framed as a cautionary tale—here’s the source of our current distress, the trivialization of politics by tabloid sensationalism. That’s true, in a limited sense, and Mr. Hart’s downfall may have had vast, if imponderable, consequences; what path might the nation have taken if he, rather than...